1. Field of the Invention
The present invention relates to a partially reflective surface antenna and, more particularly, to a partially reflective surface antenna that includes a reflective board composed of arrays of microstrip antennas, and has the advantages of low side lobe and high gain.
2. Description of Related Art
Recently, in the military or civil application fields, a partially reflective surface antenna having a partially reflective surface (PRS), which is composed of arrays of microstrip antenna, has been used frequently. These partially reflective surface antennas have low profile, and can be manufactured by using printed circuit boards (PCB).
However, the high frequency signals transmitted by these partially reflective surface antennas still have obvious levels of the side lobe portion, and the ratio of the side lobe portion to the whole waveform cannot be further decreased. This drawback prevents the partially reflective surface antenna from providing high frequency signals whose energy is centralized at the portion in the main beam direction, and thus limits the transmission distance of the high frequency signals. Furthermore, the gain of the partially reflective surface antenna cannot be continuously increased as the area of the reflective board increases. Namely, when the area of the reflective board is larger than an optimum value, the efficiency (the gain of a unit area) of the partially reflective surface antenna contrarily decreases as the area of the reflective board increases. Currently, the best available efficiency of the conventional partially reflective surface antenna is only about 50%.
FIG. 1 is a schematic drawing of the conventional partially reflective surface antenna, wherein the partially reflective surface antenna 1 comprises a substrate 11 and a reflective board 12. Both of them are composed of microwave substrates made of the FR-4 materials. The reflective board 12 is supported by the first supporting rod 141, the second supporting rod 142, the third supporting rod 143, and the fourth supporting rod 144. As a result, a resonant distance between the reflective board 12 and the upper surface 111 of the substrate 11 is maintained. The length of the resonant distance is determined with relation to the design frequency of the partially reflective surface antenna 1. Besides, a rectangular notch (not shown) is formed near the center position of the substrate 11, and the rectangular notch is electrically connected to a coaxial cable (not shown) via a rectangular to coaxial adapter to transmit or receive the high frequency signal.
When the partially reflective surface antenna is in the “transmitting state”, the high frequency signal is reflected back and forth between the substrate 11 and the reflective board 12. Later, due to the “partially reflection” effect of the reflective board 12, the high frequency signal eventually passes through the reflective board 12 and then the high frequency signal will pass through the reflective board 12 and will be transmitted outwardly by the partially reflective surface antenna 1. The length and width of the reflective board 12 are both 12.9 cm, and a plurality of microstrip reflective units 13 are disposed evenly on the upper surface 121 of the reflective board 12. The length and width of the microstrip reflective units are both 12 mm, and the distance between two adjacent microstrip reflective units is 1.1 mm.
As depicted above, even though the conventional partially reflective surface antenna 1 can properly adjust the arrangement of the microstrip reflective units 13 disposed on the upper surface 121 of the reflective board 12 (i.e. adjust the distance between two adjacent microstrip reflective units 13) to improve the signal to noise ratio (S/N ration) and the directivity of the high frequency signal it transmits. However, the ratio of the side lobe portion of the high frequency signal transmitted by the conventional partially reflective surface antenna 1 cannot be further decreased and the gain of the conventional partially reflective surface antenna 1 cannot be further increased, either.
Therefore, it is desired to have a partially reflective surface antenna being able to provide the advantages of low side lobe portion ratio and high gain, in order to improve the efficiency of antenna module in a wireless communication system.